System for recycling char in iron oxide reducing kilns

ABSTRACT

A method and means for improving the efficiency of the process for directly reducing ore containing iron oxide in a rotary kiln using a solid carbonaceous reducing agent, such as coal, introduced from the ore feed and discharge ends of the kiln, as both fuel and reductant, is disclosed wherein the charred coal or char found in the discharge product is recycled into the process at the discharge end of the kiln rather than the feed end as in the prior art. In particular, the recovered char, both coarse and finer particles, are transported to a recycle bin from which they are returned at a preselected rate to the kiln process by being injected along with the coal blown into the discharge end of the kiln. Alternatively, the recycle char alone may be fed without any coal at the discharge end of the kiln.

BACKGROUND OF THE INVENTION

The present invention relates to a process for directly reducing orecontaining iron oxide in a rotary kiln using a solid, carbonaceousmaterial, such as coal, as both fuel and reductant and, moreparticularly, to a method and means for recycling char to the kiln in animproved manner.

Many different methods have been suggested and used for carrying out thedirect reduction of ores containing iron oxide using carbonaceousmaterials, particularly coal, as both the heating agent and reductant ina rotary kiln. For example, in some of these processes the coal is fedinto the kiln through the discharge end by mechanical or pneumaticmeans, such as respectively disclosed in U.S. Pat. No. 3,113,859 toMoklebust and U.S. Pat. No. 3,505,060 to Heitmann, and in some it is fedat the center of or along the kiln, such as disclosed in U.S. Pat. No.3,206,299 to Senior et al. However, considerable problems have beenencountered with these various approaches, many of which problems havebeen overcome by the process disclosed in U.S. Pat. No. 3,890,138 toHockin. In this latter process a portion of the coal is injected orblown from the discharge end of the kiln, and the remaining portion ofthe coal is fed with the ore at the charge feed end. The kiln isgenerally divided into a reducing zone toward the discharge end and apreheat zone toward the feed end, and the coal is distributed in thekiln in such a manner that the amount of coal injected from thedischarge end is sufficient to aid in controlling the temperatureprofile throughout both zones of the kiln.

Although the process of Hockin was developed particularly for use inreducing ilmenite, it has been found that the dual end coal feedingtechnique improves upon the other coal feeding methods used in reducingiron ore to sponge iron in rotary kilns with the direct reductionprocess, and the present invention relates to improvements in thislatter process when dual end coal feeding is used.

It has been the practice in the art when directly reducing iron ores tosponge iron in a rotary kiln using coal as the reductant, to recover andrecycle the charred coal found in the discharge product. This charredcoal or char is separated from the metallized portion of the product andcollected in a char recycle bin from which it is fed at a selected rateinto the feed end of the kiln along with the coal and ore. A problem isproperly feeding the char at the feed end of the kiln is presented by acombination of the low bulk density of specific gravity of the charparticles and the considerable velocity of the exhaust gases from thekiln which may range in magnitude from 10 to 50 feet per second. Underthese conditions much of the light and small particle size recycle charwill be carried off into the waste gas system, and although againrecovered and recycled from the waste gas, much of the char may becontinuously reintroduced at the feed end, ground down, and eventuallylost from the process into the waste gas system. Of course, the morechar that can be effectively recycled, the less coal need be supplied torun the process, and thus char recycling is important to processefficiency.

The present invention describes an improvement in the process of thisgeneral type, particularly directed to reducing ore containing ironoxide, wherein the char recovered from the discharge product is recycledto the kiln by being injected into the discharge end of the kiln with orwithout blown coal.

SUMMARY OF THE INVENTION

The present invention is directed to efficiently operating a rotary kilndirectly reducing iron ore using coal fed into its opposite ends as boththe fuel and reductant and involves the feeding of recycle char,recovered from the discharge product of the rotary kiln, back into thekiln at the discharge end along with blown coal. In such kilns the oreand a portion of the coal are introduced at the feed end, and theremainder of the coal is injected from the discharge end with air alsobeing injected at spaced intervals along the kiln to promote combustion.The charge near the feed end is preheated over a zone extending forabout a third of the kiln length, and reduction occurs in the bed forthe remainder of the length. The invention takes advantage of the factthat, firstly, the velocity of the gases at the discharge end of such akiln is much lower than that of the exhaust gas at the feed end, andthus prior art feed end losses of fine char are avoided, and furtherthat returning the char directly to the bed in the reducing zone at thedischarge end avoids problems that occur with the char in the preheatzone, resulting from it being ground into fines and chemically alteredbefore reaching the reducing zone.

The rates of feeding of the char and the coal for injection into thedischarge end may be appropriately adjusted to an optimum continuouscondition, and it has been found that with this technique the feed endcoal requirement can be reduced by as much as 15 to 20%, thusconsiderably improving the efficiency of the entire direct reductionprocess. Contributing to this efficiency is the fact that not only thecoarse char recovered from the discharge product, but also finer charwhich may be recovered from the product waste may be recycled in thismanner. Alternatively, the recycle char alone may be fed without anycoal at the discharge end of the kiln.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a direct reduction plant utilizing arotary kiln for the production of sponge iron and generally illustratingthe process of the prior art.

FIG. 2 is a diagrammatic view of a part of the direct reduction plantshown in FIG. 1 and illustrating the modifications in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A flow diagram of a direct reduction plant for the production of spongeiron, of the type utilizing a rotary kiln operated generally in themanner disclosed by Hockin in U.S. Pat. No. 3,890,138 is shown inFIG. 1. The plant comprises an array of feed bins respectivelyincluding: a bin 1 for supplying ore, typically in the form of ironoxide pellets or natural lump ore; a bin 2 for providing limestone ordolomite for sulfur control; a bin 3 for providing a suitablecarbonaceous reducing agent, typically in the form of coal of less than1 inch nominal diameter particles; and a bin 4 for supplying recyclechar. the iron ore, coal, return or recycle char, and dolomite orlimestone are accurately proportioned and fed continuously as a chargeto the feed end 6a of the reduction kiln 6. A remaining bin 5 suppliescoal, typically of less than 1/2 inch nominal diameter particles to thedischarge end 6b of the rotary kiln 6, where carefully controlledquantities are injected or blown in. The coal is fed to a fireman device7 from which it is blown by means of low pressure carrier air from asuitable source 7a, through a coal injection pipe 8 which can beadjusted to achieve the optimum trajectory for this coal. A suitablesystem for the blowing and adjusting is disclosed in co-pendingapplication Ser. No. 082,138.

The reduction kiln 6 may be typically 11.5 feet (3.5 meters) in outsideshell diameter and 148 feet (45 meters) long, sloped at 3%. It may besupported on two tires and driven by a 200 horsepower variable speedD.C. motor and lined with 8 inches of refractory brick or preferablycastable refractory.

In addition to the introduction of carrier air through pipe 8, the kilnis equipped with a series of shell mounted air injection tubes 9 whichare spaced along its length and extend into the interior of the kiln fordrawing air from the outside and injecting it along the kiln axis toenhance combustion. Each of the tubes 9 is equipped with its own fan andmotor combination 10 so that the rate of injection may be properlyregulated at spaced positions along the kiln. Also, spaced along thekiln are twelve thermocouples 30 which measure the average temperatureof the charge in the kiln and of the gas.

The hot waste or off-gases exhaust from the feed end 6a of the kiln andpass into an off-gas processing or cleaning system. In a suitablecleaning system the gases may be passed first to twin refractory-linedscalping cyclones 11, which remove and recycle coarse dust and then to a57 feet high by 11.5 feet inside diameter spray cooling tower 12 wherethey may be cooled to 500° F. before passing to an 8-cell bag house 13equipped with glass fiber bags for removing the fine dust. The cleanedgases may exit via an induced draft fan and a 100 feet high stack 14.

The material discharged from the discharge end 6b of reduction kiln 6 bymeans of a sealed transfer chute consists of a mixture of sponge iron,coal char, coal ash and desulfurizing agent. This material is cooled ina rotary cooler 15 which is sealed from the ambient atmosphere, fittedwith lifters, and cooled externally with water. The cooled mixture isthen passed from the cooler 15 to a screening system 16 and screened.The two oversized fractions are subject to magnetic separation inrespective magnetic separators 17 and 18, and the remaining fines aremagnetically separated in separators 19 and 20. Separators 17 and 18remove the coarse sponge iron to the product load out area. Thenonmagnetic fractions from the separators 18 and 19 are conveyed to thereturn char bin 4. This separation system then yields coarse and finesponge iron, recycling char and fine nonmagnetic waste.

In a plant such as shown and described in connection with FIG. 1, withproper control of combustion conditions over the kiln bed and reductionconditions in the bed, high rates of heat transfer and optimumutilization of kiln volume, with metallization consistently in a 90%-95%range, can be achieved.

Regarding the process itself, directly reducing iron oxides in therotary kiln using solid carbonaceous material derived from coal as thesource of heat and reductant involves complex gas/solid reactionmechanisms. Overall reduction kinetics are affected by bed temperature,solids particle size, carbon/iron oxide ratio in the bed, reactivity ofthe char produced from the coal, the effect of catalysts on charreactivity, and reducibility of the iron oxide. Control of the rate ofheat transfer to the bed and control of bed temperature are criticalparameters for steady operation of the kiln so that stable processchemistry and kinetics result.

The properties of the coal and char used in the process are afundamental consideration in the design and metallurgical performance ofdirect reduction kilns. Physical losses of fixed carbon or char from theprocess occur in the baghouse dust, and the fine char in the nonmagneticfine waste from the screening and separation system. These losses arelargely dependent on the char strength. In addition, to maintain thecorrect carbon level in the kiln discharge, the char must havesufficient strength to resist complete degradation in passing throughthe kiln. Degradation of the char in moving from the feed end to thedischarge end in the kiln bed may occur to such an extent that the totalcoal feed rate must be increased to compensate for fine char losses, andthus a relatively high fixed carbon consumption per unit weight of ironfed may result.

Now it will be seen in FIG. 1 that the char from the return char bin 4in the prior art was fed along with the ore, coal and dolomite orlimestone, as the charge to the feed end 6a of the reduction kiln 6.However, as previously explained, the high velocity of the exhaust gasesfrom the feed end, ranging in magnitude from 10 to 50 feet per second,tend to carry off the lighter and small particle size recycle char intothe waste gas system, and although they may again be recycled from thewaste gas cyclones 11, a continuous recycling of a portion of the charat the feed end will tend to occur. Further, a degradation of the charin passing through the kiln results in fine char being discharged at thedischarge end, which becomes fine waste.

In accordance with the present invention, the prior art plant of FIG. 1may be modified for improved operation in the manner shown in FIG. 2,that is, the char in the discharge product is recycled by being injectedwith the blown coal into the discharge end 6b of the kiln 6. Moreparticularly, the nonmagnetic fractions separated by the separators 18and 19 from the discharge product constitute the recycle char and areconveyed to the recycle char bin 4'. In addition, further separation maybe carried out on the waste from separator 20 by a suitable fineseparator means 21 and the separated fine char included in the charconveyed to the recycle bin 4'.

The char in the recycle storage bin 4' may then be fed from the bin tothe discharge end of the kiln and separately injected therein with orwithout the blown coal. In the latter instance all of the coal would befed at the feed end. Preferably, however, the char is mixed with thecoal fed from bin 5 prior to being fed to the fireman device 7. The charthus mixed is injected or blown along with the discharge end coal intothe kiln through the discharge end. The rates of feeding of the char andthe blown coal may both be appropriately adjusted by control of theirrespective weigh feeders to achieve a proper mixture for optimumcontinuous operation. Suitable rates of feeding to achieve optimumprocess conditions will be readily determined by those skilled in theart in accordance with the various grades of coal used as the fuel andreductant. In any event whether the char is fed alone, separately withthe blown coal, or mixed with the blown coal into the discharge end ofthe kiln, the rate of feeding of the char should be accuratelycontrolled and proportioned by adjustment of the char weigh feeder.

It has accordingly been found that such recycling of the char into thedischarge end of the kiln significantly improves the efficiency of thereduction process. Firstly, a much greater percentage of the recycledchar is consumed in the reducing process, so that less coal is needed toproperly run the kiln. A further contribution to improving efficiencyresults from the fact that the lower gas velocities at the discharge endof the kiln permit much finer char to be fed into the discharge end thanat the feed end, so that finer char previously wasted may now berecovered, along with the coarser char from the product waste, andrecycled. Additionally, returning the char directly to the bed in thereducing zone avoids the problems that occur in the preheat zoneresulting from the char being ground into fines and chemically alteredbefore reaching the reducing zone.

What is claimed is:
 1. In the process for reducing materials containingiron oxides using a solid carbonaceous reducing agent as the source ofreductant and fuel in a rotary kiln wherein part of said reducing agentis added from the discharge end and the remainder of said reducing agentis added at the feed end, and wherein the charred reducing agent isremoved from the kiln discharge product and recycled, the improvementcomprising the steps of:returning the recycled charred reducing agent tothe discharge end of the kiln; and injecting said charred reducing agentinto the kiln through the discharge end for deposition on the chargebed.
 2. In the process of claim 1 wherein said part of said reducingagent added at the discharge end is pneumatically blown into the kilnand the recycled charred reducing agent is mixed with the blown part ofthe reducing agent when being fed to the kiln.
 3. In the process ofclaim 2 wherein the rates of feeding of the discharge end part of thereducing agent and the recycled charred reducing agent into the mixtureto be blown into the kiln are determined by the quality of the solidcarbonaceous reducing agent being used in the process.
 4. In the processof claim 1 wherein both coarse and fine charred reducing agent recoveredfrom the kiln discharge product are recycled and returned to the kiln byinjection through the discharge end.
 5. In the process for directlyreducing materials, such as ore, containing iron oxides using a solidcarbonaceous material as the source of reductant and fuel in a rotarykiln having a feed end for receiving the ore and at least a portion ofsaid carbonaceous material therethrough and a discharge end fordischarging the product therefrom and wherein the charred reductant isremoved from the discharge product and recycled, the stepscomprising:removing the charred reductant from the solid materialsdischarged from the kiln and collecting it at a receiving location;conveying the collected charred reductant from said receiving locationat a controlled rate to the discharge end of the kiln; and returning thecharred reductant to the kiln by injecting it through the discharge endonto the charge bed.
 6. A process as in claim 5 wherein all of the solidcarbonaceous material is fed with the ore at the feed end and all of thecharred reductant is fed through the discharge end.
 7. A process as inclaim 5 wherein the charred reductant is mixed with and blown along witha portion of the solid carbonaceous material through the discharge endof the kiln.
 8. In the process of claim 5 wherein both coarse and finecharred reductant are returned to the kiln by injection through thedischarge end.
 9. Method of reducing iron oxides to sponge iron using asolid carbonaceous reducing agent comprising the steps of:feedingmaterial containing iron oxide to be reduced and at least a portion ofthe solid carbonaceous reducing agent into the feed end of a rotary kilnin which the solid carbonaceous reducing agent acts as the source offuel and reductant to reduce the iron oxide; collecting the solidmaterials discharged from the discharge end of the kiln and separatingout the charred reducing agent; and returning the charred reducing agentto the kiln by feeding it through the discharge end onto the charge bed.10. Method as in claim 9 wherein the charred reducing agent is returnedto the kiln by being blown onto the charge bed.
 11. Method as in claim 9wherein both the coarse and the fine charred reducing agent areseparated out and returned to the kiln.